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Abstract

The late Cretaceous to early Cenozoic hinterland of the North American Cordillera is widely recognized as an orogenic plateau. Although most studies suggest that this highland was in place by the Eocene, the timing and underlying drivers of the plateau’s development and eventual demise are not well known. To better understand this evolving orogenic system, this study focuses on constraining the age, provenance, and depositional lag-time of Eocene sediments along the California continental margin. These sandstones were sourced, in large part, from the western margin of the highland and may therefore offer clues to its development. A total of 2,163 new detrital zircon U-Pb ages were obtained from 16 samples to confirm the published provenance of the sandstones and/or recover new grain-age populations. The U-Pb ages reveal the major sources for zircons: Permian-Triassic, Jurassic, and Cretaceous arc rocks, the Paleo- and Mesoproterozoic Mojave and Mogollon Highlands, and the Idaho Batholith/Challis Volcanic Center. Samples north of the Transverse Ranges display a stronger arc signature, while samples south of them were sourced from the Mojave and Mogollon Highlands. 159 new (U-Th)/He ages obtained from a subset of the zircons analyzed from each sample, record either one or two significant age peaks. The majority of samples have a strong latest Cretaceous cooling signature; however, most samples also record Eocene cooling. The six samples from the Santa Ynez Mountains and from Vacaville area contain cooling ages that likely reflect post-depositional resetting of samples and are not considered for further interpretation. Observations from the (U-Th)/He dates suggests that most samples with probable sources inboard of the Sierra Nevada record short lag-times, indicating little to no delay between source exhumation and sediment deposition. Overall, the (U-Th)/He dates suggest that hinterland sediment source regions were affected by major pulses of exhumation in the late Cretaceous and the Paleocene to Eocene. Late Cretaceous exhumation fits well with the timing of Sevier deformation, but rapid exhumation in the Eocene is not readily explained by the tectonic framework of the region. We hypothesize that Laramide-related tectonism in combination with rapid erosion due to changing climate are responsible for this Eocene exhumation.